Electrical power sources, such as batteries or fuel cells, are made up of an array of electrochemical cells. The state of charge (SOC) of the electrochemical cell (EC) indicates the amount of usable energy stored within the EC at a given time. It is analogous to the amount of fuel in a fuel tank. In order to improve EC life and overall EC performance, the EC must be maintained at an optimal SOC. The state of health (SOH) of an EC indicates the physical condition of the EC. The SOH is based on observing particular EC parameters to determine the extent of unobservable, internal damage or degradation.
An EC can be considered a system that has current and heat flow as inputs and responses of voltage and temperature. The voltage response of the EC to current stimulation is described using a combination of voltages and a current source. These voltages include an equilibrium potential, voltage resulting from hysteresis effects, voltage resulting from ohmic resistance drops, and voltage resulting from dynamics in the EC, or double-layer voltage. Each of these voltages can be described by either an algebraic function, a differential equation or a convolution integral. The current source is indicative of a self-discharge current of the EC.
During use, resistance, equilibrium potential, voltage resulting from hysteresis effects, voltage resulting from ohmic resistance drops, double-layer voltage, and self-discharge current are not directly measurable. These values influence the SOC and the parameters that indicate the SOH of the EC. Because they are not directly measurable, it is often difficult to accurately determine the SOC and SOH of the EC. Therefore, it is difficult to maintain the EC at an optimal SOC or to determine when the SOH has degraded to a point where EC performance is significantly effected.